There are plenty of sites that will give you a scientific measurement, normally the watts needed to roll the tyre at certain pressures and speeds. Obvious the fewer watts, the fewer watts "wasted" flexing the tyre and more available for movement.
There is an argument that there is a "sweet spot" somewhere between thick and thin, because the thinner the tyre the higher the psi needed - which means the rider is moving up and down more, which requires energy in itself - at some point it is more efficient to flex the tyre then wobble you up and down.
My personal view is that it is understated, because it is normally calculated by those factoring it as if the entire weight of the rider is being shook up and down.. Of course if you are riding over a rough bit of road you tend to use your legs to dampen the vibrations, so minimising some of this. However, wider tyres will of course be more comfortable.
In the end, it's down to individual preference but you will probably make performance gains on identical tyres down to at least 25c.